Use of national-scale data to examine human-mediated additions of heavy metals to wetland soils of the US

There are few available, large-scale, chemical indicators that can be used to rapidly detect the potential for anthropogenic impacts to wetlands. Soil concentrations of 12 heavy metals that have been linked to various anthropogenic activities were measured in samples collected from the uppermost horizon in approximately 1000 wetlands across the conterminous US as part of the 2011 National Wetland Condition Assessment (NWCA). The heavy metals were silver (Ag), cadmium (Cd), cobalt (Co), chromium (Cr), copper (Cu), nickel (Ni), lead (Pb), antimony (Sb), tin (Sn), vanadium (V), tungsten (W), and zinc (Zn). These data were used to develop a Heavy Metal Index (HMI), an indicator of anthropogenic impacts, to report on areal extent of wetlands with human-mediated additions of heavy metals to the soils that may ultimately affect biological condition. Additionally, these data were used to report the first (i.e., baseline) wetland soil heavy metal concentrations on national and regional scales. Our data show that wetlands of the conterminous US typically have low levels of heavy metals in the soil, and that most of the elements included in this analysis occur nationally in concentrations below thresholds that relate to anthropogenic activities. However, concentrations above expected natural background (i.e., > 35 ppm) of soil lead occur nationally in 11.3% of the wetland area. Regionally, the greatest extent of wetland area with elevated soil lead concentrations was located in the Eastern Mountains & Upper Midwest followed by the West (23.9 and 18.4% of the wetland population, respectively). Our data show positive relationships between soil lead concentration and four individual landscape metrics: road density, percent impervious surface, housing unit density, and population density in a 1-km radius buffer area surrounding the site. These relationships, while evident on a national level, are strongest in the eastern US (i.e., in the Eastern Mountains & Upper Midwest and in the Coastal Plains), where the highest road densities and greatest population densities occur. Because lead can be strongly bound to wetland soils in particular, maintenance of the good condition of our nation’s wetlands is likely to minimize risk of lead mobilization.

Impact/Purpose

A team of scientists associated with EPA investigated soil concentrations of 12 heavy metals, each of which have been linked to various anthropogenic activities, at a continental scale by analyzing data from the 2011 National Wetland Condition Assessment (NWCA). Using information found in the literature, background concentrations for each of the heavy metals were designated above which indicate human disturbance to a wetland site. The soil concentration data and threshold information were used in combination to develop a Heavy Metal Index (HMI) and report on the area of wetland (i.e., extent) impacted by low, moderate, and high HMI stressor-levels. Additionally, mean soil concentrations of each of the 12 heavy metals were investigated. Major findings were: • Wetlands of the conterminous US typically have low stressor-levels of heavy metals in the soil. • Of all the heavy metals measured, lead (Pb) occurred most frequently concentrations that indicate human disturbance, with almost a quarter of the wetlands in the Midwest affected by elevated soil lead concentrations. • Soil lead concentrations are positively correlated with road density, percent impervious surface, housing unit density, and population density surrounding the site. Understanding soil chemistry in our nation’s wetlands and how it relates to landscape disturbances can provide useful information to managers and policy makers as well as a way to set priorities for management actions in specific locales. This study also contributes to work being done under SSWR Task 3.01A, Subtask 1.1.

Citation

Nahlik, A., K. Blocksom, A. Herlihy, M. Kentula, T. Magee, AND Steve Paulsen. Use of national-scale data to examine human-mediated additions of heavy metals to wetland soils of the US. Springer, New York, NY, 191:336, (2019). [DOI: 10.1007/s10661-019-7315-5]

DOI: Use of national-scale data to examine human-mediated additions of heavy metals to wetland soils of the US